Characterization of Outdoor Malaria Transmission and Plant Feeding Pattern Among Anopheles Funestus and Anopheles Gambiae Mosquitoes From Selected Ecologies of Kenya

Abstract

Outdoor malaria transmission remains a major hindrance to effective control and elimination of the disease. However, the anopheline vector composition among outdoor biting fractions is poorly understood including associated bionomic traits especially in dryland ecosystems of Kenya. In this study, adult mosquitoes belonging to Anopheles gambiae complex and Anopheles funestus group were characterized for their sibling species composition, Plasmodium falciparum infection rates and plant feeding profiles. The mosquitoes were trapped outdoors around homesteads using CO2-baited CDC light traps through a cross sectional survey in selected dryland areas including Kerio Valley, Nguruman and Rabai. Mosquitoes from An. funestus group (n=639, 90.6%) were abundant than those of the An. gambiae complex (n=66, 9.4%) across the three study areas. Molecular identification of the sibling species in the Funestus group via PCR of the internal transcribed spacer 2 (ITS2) region and infection with non-coding mitochondrial sequence (ncMS) identified An. longipalpis C as the dominant vector with a Plasmodium falciparum sporozoite rate (Pfsp) of 5.2% (19/362) respectively. Other species were Anopheles funestus s.s. and Anopheles rivulorum with Pfsp rate of 8.7% (2/23) and 14.1% (9/64), respectively, varying among the areas. PCR of An. gambiae complex samples targeting the rDNA identified An. arabiensis and An. gambiae s.s with the former (52/66) occurring in higher proportions than the latter (14/66). An. gambiae s.s had a Pfsp rate of 14.3% (2/14) and An. arabiensis 1.9% (1/52). Additionally, six cryptic species associated with An. funestus group were uncovered and found to harbor Pf sporozoites (cumulative Pfsp rate=7.2%, 13/181). The species were identified after PCR of the ITS2 and then sequencing and phylogenetic analysis. Cold anthrone test, used to analyze the mosquitoes for evidence of recent plant feeding, found an overall low fructose positivity rate (2.8%, 19/680). Analysis of a subset of samples including the fructose-positive specimens by DNA barcoding of ribulose-1,5 bisphosphate carboxylase large chain (rbcL) gene, identified plant DNA with a 55.3% (n=262) success rate. Plant DNA sequences were successfully generated from 62.3% (71/114) samples, implicating acacia plants as the predominant plant fed upon by mosquito vectors from Kerio Valley and Rabai. The findings from this study indicate that outdoor malaria vector species composition is dominated by lesser known species which potentially play a role in malaria transmission. The results have implications for persistent malaria transmission and effective malaria control in the study areas. Additionally, findings from the study indicate that cold anthrone test for fructose, grossly underestimates the extent of plant feeding in disease vectors. While plant feeding appears to differ among the species, the basis for seeming preference on acacia requires additional research.